1. Field of the Invention
The present invention relates to an information recorder and its control method, and more particularly to an information recorder which handles a removable sequential access medium such as a magnetic tape, and its control method.
2. Description of the Related Art
The sequential access medium such as the magnetic tape is used for data storage, e.g., database backup, in many cases. Generally, the database backup comprises copies of all data, and journal data such as information before updating and information after updating for update information of the database. In an on-line system, the journal data which is the update information of the database by sequentially executed transactions is very important for restoring data when troubles occur, and has been handled to enable strict maintenance of data integrity so much so that writing is considered to be completed at the time of duplexing on the magnetic tape in consideration of troubles of the magnetic tape medium. Recently, a requirement of real-time duplexing has become not so strict at the time of writing in the magnetic tape since the journal data is not directly written in the magnetic tape so often, but it is in many cases copied to be stored in the magnetic tape after being written in a magnetic disk or the like. However, such backup data is very important data which becomes necessary for restoration when data is broken down by troubles of the device or disasters. Thus, in order to prevent troubles such as impossibility of reading due to a medium trouble after writing in the magnetic tape, or in order to store the data at a remote place to prepare against disasters, a process of duplexing data is still carried out in many cases.
For original and copy duplexing of the data on the magnetic tape, there have been presented, for example, a method for increasing efficiency by reducing the number of used magnetic tape devices (JP-A-S58-166472) and a file creation method for facilitating management of data duplexed by a smaller number of magnetic tape devices (JP-A-H04-344922).
FIG. 16 shows a conventional example of a system and method for the original and copy duplexing of the data on the magnetic tape. As shown in FIG. 16, the system is on the assumption that writing is carried out in two magnetic tape devices 902 and 903 from a host system 901. This system has an advantage that all including interfaces from the host system 901 to the magnetic tape devices 902 and 903 can be duplexed, and has been widely used. However, the host system 901 must issue a writing command twice, which imposes a load on the host system 901.
In technical fields of the duplexing of the data on the magnetic tape, recently, a concept of a storage area network has spread especially, and there has been increased adoption of a system and method based on the concept. FIG. 17 shows a conventional example of a system using the storage area network. As shown in FIG. 17, a plurality of magnetic disks 912, 923, 914 and a plurality of magnetic tapes 915, 916 are connected to the same interface 911. In the case where the plurality of peripheral devices 912 to 916 are connected to the same interface 911, if the host system 901 issues the same writing command twice, not only a load on the host system 901 but also a usage rate (busy rate) of the interface 911 are increased.
To deal with such a problem, the inventors discloses an information recording/reproducing system which includes a function of mirroring by making two physical drives which seem one logical drive (JP-A-2002-132559). Because of the mirroring by making the two physical drives which seem one logical drive, the host system needs to issue a writing command only once to enable writing of data in a plurality of recording media. Accordingly, data duplexing can be achieved without increasing the load on the host system or the usage rate of the interface.
Needless to say, by employing such a form, compared with a system similar to that shown in FIG. 16 for carrying out writing in the two magnetic tape devices from the host system, redundancy is lost because of nonduplexing of the interface portion. However, as described above, the requirement of data duplexing to be carried out simultaneously with the writing in the magnetic tape is not so strict now, and only electric signals are normally transferred through the interface portion. Compared with troubles in a drive such as a magnetic tape device including a movable potion, e.g., a motor, and a removable medium such as a magnetic tape medium in which deterioration occurs due to a secular change, and physical stress is applied for each use, a probability of trouble occurrence is considerably smaller to be ignored.
However, in the case of mirroring by making the two physical drives which seem one logical drive, it is assumed that upon completion of writing from the host, two rolls of recording media of identical contents are generated. Thus, if one process is interrupted by a trouble of the physical drive or the recording medium, the occurrence of the trouble must be notified to the host system, consequently increasing a trouble rate by about twice.
In connection with the aforementioned problem, JP-A-H06-124169 discloses a method which enables continuation of a process by replacing a medium of a trouble with a spare medium prepared beforehand, and restoring data on the spare medium based on a content of a medium of no troubles when the medium trouble occurs in a duplexed autochanger.
However, this method supposes a case of a medium to be randomly accessed such as an optical disk. In a sequential access medium such as a magnetic tape, a tape position cannot be changed on-line and, even if it is replaced with a spare medium, the method disclosed in the JP-A-H06-124169 cannot be applied to restore a duplexing process.
An object of the present invention is to provide a high-availability duplexing or multiplexing information recorder for handling a sequential access medium such as a magnetic tape, where a process is not interrupted by a trouble of a drive or the medium.